1. Field of the Invention
The present invention relates to a water-repellent filter member and a method of manufacturing the water-repellent filter member, as well as to a waterproof instrument using the water-repellent filter member, and a gas sensor using the water-repellent filter member.
2. Description of the Related Art
Among instruments having an internal space, certain instruments allow air communication between the internal space and the exterior thereof via an air vent or the like. Unless appropriate measures are taken against entry of water, there is a risk of water entering into the internal space via the air vent in association with entry of the air. Certain instruments cannot tolerate water entering into the internal space because of their applications or characteristics and thus employ waterproofing measures, thereby becoming waterproof instruments. Accordingly, in an instrument that must be waterproof and allow air communication between the exterior thereof and the internal space, the air vent must also be waterproof and allow air communication.
An example of such a waterproof instrument is the oxygen sensor disclosed in Patent Document 1. The oxygen sensor has an oxygen detection element configured such that an internal electrode is formed on the inner surface of an axially extending tubular solid electrolyte element having a closed tip end and an open rear end, whereas an external electrode is formed on the outer surface of the solid electrolyte element. The air, which serves as a reference gas, is brought into contact with the inner surface of the solid electrolyte element, and an exhaust gas, which is a gas to be measured, is brought into contact with the outer surface of the solid electrolyte element. By utilizing electromotive force induced by an oxygen concentration cell effect between the internal and external electrodes, oxygen concentration is detected.
The oxygen sensor includes a metallic shell adapted to fix the oxygen detection element, and a sleeve portion whose tip end portion is fixedly attached to the rear end of the metallic shell and which extends axially in a direction opposite the metallic shell so as to cover a rear end portion of the oxygen detection element. A grommet having an air through-hole formed at its center is fit into the open end of the sleeve portion, and the sleeve portion and the grommet are airtightly sealed together. Accordingly, the internal electrode is accommodated in an internal space defined by the metallic shell, the sleeve portion, and the grommet. The air is introduced into the internal space through the air through-hole of the grommet.
If introduction of the air into the internal space of the oxygen detection element is accompanied by water, there is a risk of a short circuit occurring between metallic terminals connected to the internal and external electrodes of the oxygen detection element and a short circuit between the above-mentioned metallic terminals and metallic terminals for supply of electricity to a heater accommodated, together with the oxygen detection element, in the internal space. Such short circuit potentially results in a failure to properly detect electromotive force induced by the oxygen concentration cell effect.
In order to cope with this problem, the oxygen sensor can employ a resin sheet of a porous, fibrous structure having air permeability and water repellency. Specifically, the resin sheet is pressed into the air through-hole by use of an open-ended tubular metal member, thereby forming a water-repellent filter structure. The water-repellent filter structure prevents entry of water into the internal space from the exterior of the oxygen sensor while maintaining air communication through the air through-hole. By virtue of this structure, the oxygen sensor can have high reliability. Namely, while the internal electrode of the oxygen detection element is in contact with the air, no water enters the internal space thereof.
[Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. 2000-193632 (FIGS. 1 and 3)
3. Problems to be Solved by the Invention
However, as mentioned above, when the resin sheet is pressed into the air through-hole using the tubular metal member so as to form the water-repellent filter structure, wrinkled portions of the resin sheet are present between the tubular metal member and the wall surface of the air through-hole.
Since any gap between the resin sheet and the wall surface of the air through-hole risks the entry of water therethrough, the outside diameter of the tubular metal member and the diameter of the air through-hole are rendered substantially equal to each other. Accordingly, when the resin sheet, together with the tubular metal member, are pressed into the air through-hole, friction arises on the resin sheet held between the internal tubular metal member and the air through-hole and generates high insertion resistance. The resin sheet is pressed into the air through-hole against the insertion resistance by inserting the tubular metal member.
Since the resin sheet and the tubular metal member are forcibly pressed into the through-hole, a large insertion force or load is applied to the resin sheet, particularly on a portion (shoulder portion) corresponding to an edge portion of the leading end of the tubular metal member. This involves the risk of cracking or breakage at the shoulder portion of the resin sheet, potentially resulting in impaired reliability and yield. Also, since application of a large load is required for insertion, it is not an easy task to form such a water-repellent filter structure.